Hands on with Java XML filter pipelines

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Setting up a pipeline

Once you have your filter set up and compiled, you need to create a pipeline for processing your XML. This should move from input XML document to the filter to the reader. You may even have multiple filters, stacked upon each other. As long as input comes first, and your reader (with application-specific callbacks) comes last, things work fine.

The following listings show how to set up a program to use filters. To do this we have defined a simple XML application (called SimpleXMLApplication). This is a standard SAX Content Handler (it extends the DefaultHandler to obtain the default behaviour). This application merely echoes, to the standard output, the XML passed to it (with suitable indentation).

The XMLFilterTest test harness class links the SimpleXMLFilter and the SimpleXMLAPplication together. Notice that because the one or more filters must sit between input source and the reader, all the operations that you would normally invoke on the reader are invoked on the filter. It then delegates any data that passes through the filter to the reader.

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Also note that we obtain the XMLReader form the root parse and set that as the "parent" of the filter. We then link the filter with the application by making the application the content and document handler of the filter.

The effect of running the XMLFilterTest on a simple XML document is presented below:

Data Pollution

However, a word of warning if you use a filter to remove some elements form the data input to the next element in the pipeline. It is all too easy to pollute the data being sent on. For example, consider the case where you don't delegate in the startElement() method for certain data, but forget to do the same in endElement(). The result would be that some elements would never be reported as starting, but would be reported to the reader as ending. This would cause, in the best case, program errors, and in the worst case, data loss or corruption in your application.

Real world use

As an example of a tool that makes extensive use of pipelines of filters, consider the DeltaXML XML diff tool. This uses XML filters to pre and post process XML data before performing XML comparisons, synchronization operations and patches. To download a time limited copy of DeltaXML see the DeltaXML web site here. ®